`MergeRequest#source_sha` is expected to return the sha of the source
branch last commit.

But when a open Merge Request has no diff (e.g. all commits have already
been merged to the target branch), `merge_request.source_sha`
incorrectly returns `nil`.

This was un-noticed before – but now that !2217 has been merged,
it makes `Gitlab::Git::Commit.between` raise an
"Unexpected nil argument" exception.

This fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available.

Fix requirements.md link and improve PostgreSQL commands

 - Point requirements.md link to the correct location

 - Link to gitlab installation.md document instead of github

 - Use psql command switch to run specified SQL commands for creation of user and database permissions.
    -  Instead of requiring a comment like `# Do not type the 'template1=#', this is part of the prompt` it may be easier to just use the command switch with `psql` to execute the SQL commands.


*forgot [ci skip] :(*

See merge request !3176

Return an empty Array when there aren't lines to parse.

Fixes #14306

See merge request !3243

Support Wiki with UTF-8 page name

## What does this MR do?

Support Wiki with UTF-8 page name.

See https://github.com/gollum/gollum/pull/929

## Why was this MR needed?

Relax constraints for wiki slug in aac65984.

It allows to create a wiki with UTF-8 name, but creating a wiki with UTF-8 name causes 500 error.

Creating a wiki with UTF-8 name once, then creating or updating wiki with ascii name also cause 500 error i.e. no one can create and update any wiki pages in the project.

## Workaround

1. Go to `https://DOMAIN/repo/wikis/git_access` -> this page display the link to git clone
2. Clone the wiki repo
3. Find the page with UTF-8 name
4. Rename or Delete these files
5. Commit and push

## What are the relevant issue numbers?

- #13979
- #13891
- #13698
- #13603
- #13317
- #12906
- #12825
- #10945 

## Todo

- [x] Waiting for 'gollum-rugged_adapter' that support rugged v0.24.0 release

'gollum-rugged_adapter' gem doesn't allow to install rugged v0.24.0 (it's still beta version),
but 'gitlab_git' gem depends on rugged v0.24.0b13.

So it can't install both 'gollum-rugged_adapter' and 'gitlab_git' now.


See merge request !2999

A worker deletes a user, so the request doesn't time out

Fixes #13261


See merge request !2855

Fix Capybara 2.6.2 deprecation warnings

See merge request !3236

Bump Git version requirement to 2.7.3

[ci skip]

See merge request !3240

Fixes label on "Assignee" dropdown

Fixes #14301 

See merge request !3239

Fixed overflow on users name in dropdowns

Closes #14289

![Screen_Shot_2016-03-15_at_17.38.09](/uploads/803341031c468069709afd99a6c540be/Screen_Shot_2016-03-15_at_17.38.09.png)

See merge request !3235

Fixes #13261

Allow subscribing to labels

This implement #12743 and supersedes !2799 (thanks @timothyandrew!).

- Subscribe (and unsubscribe) for labels
- When an issue/merge request is created, notify all subscribers of all its labels
- When an issue/merge request is edited, notify all subscribers of all its newly-added labels

## Done

- [x] Verify that a user is signed in to subscribe/unsubscribe from a label.
- [x] Merge conflicts
- [x] Integration tests
- [x] `issuable.subscribed?` should check subscribers and participants
- [x] When an issuable is relabeled, notify subscribers of the *newly added* labels only
- [x] Screenshots:

### Labels page

![Screen_Shot_2016-03-07_at_19.05.44](/uploads/3e69064e9e1a06ee2e27c778776c1407/Screen_Shot_2016-03-07_at_19.05.44.png)

### HTML email

![Screen_Shot_2016-03-07_at_19.07.36](/uploads/e484e06366a738385f1f6d71b52eecf7/Screen_Shot_2016-03-07_at_19.07.36.png)

### Plain text email

![Screen_Shot_2016-03-07_at_19.07.50](/uploads/dc05f710a8f7ab5eae9f72aa2110e741/Screen_Shot_2016-03-07_at_19.07.50.png)

PS: I've set the milestone to 8.6 since it's getting late for 8.5...

See merge request !3115

Closes #14289

1. Make the "subscribed" text in Issuable sidebar reflect the labels
   subscription status

2. Current user mut be logged-in to toggle issue/MR/label subscription

[ci skip]

CSS style guide

Working towards what was discussed in #13552, this adds the [SCSS Linter gem](https://github.com/brigade/scss-lint) for style guide conformity in CI.

TODO:
- [x] Agree on and write SCSS Style Guide Documentation.
- [x] Document the `scss-lint` config file.
- [x] Figure out how best to run this in CI, right now it's taking longer than I would think it should.
- [x] Use CSSComb for auto-correction (Maybe just include a CSSComb config file and have developers run the node package manually if they're interested in using it?).

My logic for not using CSSComb in the first place is that, AFAIK, we don't currently require Node/NPM, even in the dev environment. Maybe I'm wrong about that? `scss-lint` is a Ruby implementation of a similar concept, but it doesn't include autocorrect.

Is there a way we can run get GitLab CI to run `scss-lint` without having to put together the full GitLab application? Seeing as it's just static analysis, it seems like a waste of time/resources.

/cc @jschatz1 @rspeicher

See merge request !3069

Remove parentheses from if statement

Otherwise Rubocop will give a warning after we upgrade it.

As discussed in !3197.

[ci skip]

See merge request !3231

Otherwise Rubocop will give a warning.

As mentioned in !3197.

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

Ignore eager loading in Project.search UNION

This fixes issues such as filtering groups by names on pages such as
https://gitlab.com/dashboard/groups.

See merge request !3229

Set link title for todos. Closes #14042

See merge request !3161

See merge request !3218

Fix typo in Ruby CI test and deploy example

[skip ci]

See merge request !3201

Revert the avatar cropping feature

Reverts !2951 since the added 'mini_magick' dependency should be replaced by client-side croppping

See merge request !3223